With the advance of electric equipment toward smaller size, lighter weight and higher performance, the semiconductor mounting technology has changed from the pin mating type to the surface mounting which now becomes the mainstream. One bare chip mounting technology is flip-chip (FC) mounting. The flip-chip mounting is a technique of providing an LSI chip on its circuit pattern-bearing surface with several to several thousands of electrodes, known as bumps, of about 10 to 100 microns high and joining the chip to electrodes on a substrate with a conductive paste or solder. Then the sealing material used for the protection of FC devices must penetrate into gaps of several tens of microns defined by bumps between the substrate and the LSI chip. Since the resin underfill system requires to fill very narrow gaps with the resin, voids and shorts are often left. It is thus strongly desired to quickly detect such defects in a continuous manner so that defective parts may be eliminated.
In the prior art, an ultrasonic flaw detector is generally used for the detection of defects in devices sealed by the resin underfill method. Although the ultrasonic flaw detector can detect very small defects, it is unamenable to automatic detection, imposing a limit on the quickness of defect detection.
For conventional semiconductor devices, a soft x-ray detector is widely employed for the purpose of detecting wire flow or wire breakage. Where flip-chip type semiconductor devices are sealed with an underfill material using silica as an inorganic filler, the resin layer is so thin that it may become difficult to discriminate voids from the resin with soft x-rays.